Sarny is the main character in Nightjohn. She is twelve, and people think she is "dumb", because she has "a stuck tongue", and when she was born she "come out wrong...all backwards and twixt-and-twinst". In reality, however, Sarny is very intelligent, and learns how to read very quickly.

Nightjohn is a slave who is brought to the Waller plantation. He is unique in that he once escaped to the North, but came back to teach slaves to read. Nightjohn is courageous, and self-sacrificing. He endures brutal punishment in order to give slaves what he considers to be their ticket to eventual emancipation - literacy, so that they might become aware of their situation and be inspired to fight for their freedom.

Mammy, or old Delie, takes care of the children on the plantation. She is a loving figure to the young ones who are taken from their real mothers as soon as they are born. Delie is strong, practical, and wise, maintaining an untouchable dignity even in the face of brutal and inhumane treatment at the hands of Waller.

Clel Waller is the "master" on the plantation. He is a cruel, sadistic man who treats his slaves like animals, breeding them like livestock, working them to the point of exhaustion, and feeding them from a trough, adults and children alike. Sarny correctly observes that he has power only because "he's got dogs and a whip and a gun and so can cause hurt".